Fluorescent lamp circuit with low voltage supply

ABSTRACT

A transistorized feedback oscillator circuit for igniting and operating fluorescent lamps from a 12 volt direct current power source. The primary winding of a transformer is supplied by the interrupted current output of a power transistor, and the transformer secondary winding is connected to the electrodes of a fluorescent lamp or lamps. Low temperature operation is achieved by employing a temperature compensated resistor in the divider network biasing the power transistor base. Feedback from the power transistor is isolated from the base thereof through a driver transistor permitting operation on a full wave rectified pulsating direct current. A diode is employed for protecting the circuit against reverse polarity connection and continuous fluorescent lamp filament heating is produced by the direct current supply.

United States Patent 1 Marshall et al.

[ 1 FLUORESCENT LAMP CIRCUIT WITH LOW VOLTAGE SUPPLY [75] Inventors: John J. Marshall, Grand Rapids;

David B. Dykehouse, Marshall, both of Mich.

[73] Assignee: Progressive Dynamics, Inc.,

Marshall, Mich.

[22] Filed: Nov. 26, 1972 [21] Appl. No.: 202,427

[ .Jluly 17,1973

3,435,206 3/1969 Swanson 240/51.ll X

Primary ExaminerAlfred L. Brody Attorney-Beaman & Beaman [5 7 ABSTRACT A transistorized feedback oscillator circuit for igniting and operating fluorescent lamps from a 12 volt direct current power source. The primary winding of a transformer is supplied by the interrupted current output of a power transistor, and the transformer secondary winding is connected to the electrodes of a fluorescent lamp or lamps. Low temperature operation is achieved by employing a temperature compensated resistor in the divider network biasing the power transistor base. Feedback from the power transistor is isolated from the base thereof through a driver transistor permitting operation on a full wave rectified pulsating direct current. A diode is employed for protecting the circuit against reverse polarity connection and continuous fluorescent lamp filament heating is produced by the direct current supply.

10 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION The invention pertains to an electronic circuit for operating fluorescent lamps from a low voltage direct current power source.

It has been long appreciated that fluorescent lamps are a more effective light producer than incandescent lamps with respect to electric power consumption. However, until recently, fluorescent lamp operation in portable devices was not practical in view of the high voltage requirements for lamp ignition and operation, and other electrical considerations. In more recent years circuits have been devised for fluorescent lamps wherein the lower power consumption, as compared with incandescent lamps, has been utilized to advantage, as shown in U.S. Pat. No. 3,435,206.

The light produced in a fluorescent lamp is a result of the conversion of ultraviolet energy by the phosphor on the lamp tube wall into visible light, and the ultraviolet energy is produced by the vaporization of mercury occurring in the lamp arc. Ignition of the arc and the efficiency of the visible light produced are sensitive to the temperature of the lamp, and at lower temperatures the light emitted by fluorescent lamps is substantially reduced as compared with the light emitted when the lamp is at room temperature. For instance, at 20 F. the visible light output is less than 20 percent of maximum. Thus, one of the deficiencies of portable or low voltage supplied fluorescent lamps has been with respect to the production of an efficient light at lower ambient temperatures.

In view of the higher luminous efficiency of fluorescent lamps with respect to incandescent lamps it is desirable to use fluorescent lamps with battery powered sources as the lower power consumption provides extended lamp output for a given power source. For instance, recreational vehicles such as trailers, campers and mobile homes, and many pleasure boats, require interior light fixtures'supplied from the battery powered system of the vehicle or boat, or from a separate auxiliary battery powered system, and the use of fluorescent lamps in such interior light fixtures is highly desirable in view of the lower power consumption. However, as such recreational vehicles and boats are often used under climate conditions wherein the interior temperature is well below normal room temperature, dependable and efficient fluorescent light operation for use with recreational vehicles and watercraft has heretofore been difficult to achieve.

Recreational vehicles and pleasure watercraft normally utilize l2 volt direct current electric power for interior lighting, as such lighting is often required when 1 volt alternating current is not available. In order to conserve the battery and permit the use of l volt alternating current with recreational vehicles and watercraft having 12 volt wiring systems when the higher voltage is available, it is common to incorporate a full wave rectifier in the electrical system which, when supplied with I 10 volts alternating current, produces a full wave rectified, unfiltered, direct current power. As such rectified, pulsating direct current produces a varying voltage, transistorized oscillating circuits as previously used with fluorescent lamp circuits do not operate well on full wave rectified unfiltered direct current, and this limitation of operation of previous fluorescent 2 lamp circuits has limited the use of fluorescent lamps in recreational vehicles and watercraft.

SUMMARY OF THE INVENTION It is the object of the invention to provide a fluorescent lamp circuit operating from a low voltage direct current power supply, such as 12 volts, which is of a low cost, dependable in igniting the fluorescent lamp and maintaining lamp operation, and is capable of providing efficient lamp operation at lower temperatures.

An additional object of the invention is to provide a fluorescent lamp circuit capable of operating satisfactorily on full wave rectified unfiltered direct current power wherein dependable oscillation of the transistors employed in the circuit is achieved, and dependable lamp ignition and operation is obtained. Further, is desired that the circuit in accord with the invention accommodate two fluorescent lamps, and simultaneously ignite both lamps upon circuit energization.

The circuit is of the feedback oscillator type employing a power transistor and a driver transistor imposing an isolated control current to the power transistor. The power transistor provides an interrupted current to a transformer primary winding, and the secondary winding of the transformer includes terminal leads each of which are connected to the electrodes of a fluorescent lamp. The voltage output of the secondary winding is divided between the lamps. A feedback circuit from the power transistor is isolated from the power transistor through the driver transistor in order to provide reliability and stability of the oscillation of the power transistor, and permit use of the circuit with full wave rectified, unfiltered, pulsating direct current at normal and at low temperatures.

The power transistor includes a divider network connected to its base, and this network incorporates a temperature compensated resistor having a positive temperature coefficient whereby increased biasing currents are imposed upon the transistor base at lower temperatures, and the current is reduced as the temperature increases. Such temperature compensation insures oscillation of the circuit through a wide temperature range and maintains the circuit operation at peak efficiency.

The circuit is protected against damage due to the incorrect supply polarity being applied to the wrong side of the circuit by a diode, and 'a radio frequency filter, and audio and low frequency filter are incorporated in the circuit to minimize interference with radio and television usage.

A voltage divider network is connected across the secondary winding leads to divide the transformer output between the two lamps, and a capacitor connected to the voltage divider network and connected to the positive power supply conductor insures that both lamps will ignite simultaneously.

BRIEF DESCRIPTION OF THE DRAWING The aforementioned objects and advantages of the invention will be appreciated from the following description and accompanying drawing consisting of the circuit depicting the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the circuit diagram, direct current power is supplied by the positive conductor 10, and the negative or ground conductor 12. It is intended that the circuit be particularly adapted for use with 12 volt DC power sources, and the conductors and 12 may be connected through an on-off switch, not shown, to a 12 volt battery source, or alternatively, to a full wave rectified source of unfiltered direct current power as is often utilized with recreational vehicles and pleasure watercraft when parked or docked.

Basically, the circuit employs a 2N2222 driver transistor 14 having base, collector and emitter terminals, and a 2N5037 power transistor 16 also employing a base, collector and emitter. A transformer 18 of the ferrite powder core type increases the voltage produced by the transistorized feedback oscillator circuit and supplies fluorescent lamps 20 and 22, represented by dotted lines. The lamps 20 and 22 are preferably of the watt type and each includes a terminal or electrode 24 or 24 located at the left end as illustrated in the drawing. The lamps also include interconnected filaments 26 and 26' located at the other end of the lamp in opposition to the electrodes 24 and 24, as is well known in fluorescent lamps construction. The electrodes 24 and 24' function solely as electrodes, while the filaments 26 and 26' function as both electrodes and heated filaments.

The transformer 18 includes a core 30, a primary winding 28 having a lead connected to the conductor 10, and a second lead connected to the power transistor collector. In the preferred embodiment of the invention the primary winding 28 consists of 10 turns of No. wire. The transformer secondary winding 32 includes terminal leads 33 and 35, lead 33 being coupled to the upper lamp electrode 24, and lead 35 being connected to the lower lamp electrode 24'. The secondary winding 32, in its preferred embodiment, consists of 91 turns of No. 28 wire. The transformer 18 has a natural frequency of approximately 18 to 20 kHz, which produces a high frequency output and permits small transformer geometry and small capacitor size.

A capacitor voltage divider network is connected across the secondary winding leads 33 and 35 and includes capacitors 34 and 36 connected in series having a center tap conductor 38 which is connected to the negative conductor 12. The capacitors 34 and 36 have a value of 470 PF. 1 KV, and by employing this type of circuit the four lead transformer 18 may be economically manufactured and the voltage output of the secondary winding equally divided between the lamps 20 and 22, which, in the disclosed embodiment, is 57 volts per lamp. A0.0l MF. l KV capacitor 40 is connected in the lead 33 between the voltage divider network and the associated electrode 24 to prevent direct current from flowing in the secondary winding.

A 470 PF. 1 KV capacitor 42 is also connected to the lead 33, and is connected to the positive conductor 10 by conductor 46 in order to insure that both lamps 20 and 22 ignite simultaneously.

In order to improve the operation of the lamps 20 and 22 to achieve maximum visible light emmission at lower temperatures, the series connected filaments 26 and 26 are heated upon energization of the circuit. Such heating is accomplished by connecting the filaments 26 26' in series with each other across the conductors l0 and 12 by means of conductors 44 and 46. Thus, the filaments will heat the lamps and maintain a temperature of the lamp phosphor sufficient to permit effective light emission even though the ambient temperature may be relatively low.

Biasing of the base of the driver transistor 14 is controlled by the divider network connected across conductors 10 and 12 including resistors 48 and 50. Resistor 48 has a value of 3.3K OHM, 1/2W., and the resistor 50 has a value of 100 OHM, l/ZW. The base of transistor 14 is connected to this divider network by conductor 51.

The diode 52 connected between conductor 51 and ground 12 is for the purpose of bypassing negative voltages in excess of 0.7 volts to ground in order to protect transistor 14 from reverse voltages between its base and emitter which might prove destructive. The emitter of transistor 14 is connected to ground by conductor 54. The transistor collector is connected to resistor load 56 which is of a 100 OHM 2W value, and is connected across conductor 10.

Improved oscillating circuit characteristics result from the feedback system utilized which includes a conductor 57 connected to the power transistor collector and the associated primary terminal lead. The conductor 57 is connected to the base of driver transistor 14 through conductor 51, and the capacitor 58 of a 0.047 MFD. value. In this manner capacitor 58 couples AC positive feedback voltage from the power transistor 16 to the base of the driver transistor 14, and this type of interconnection isolates the feedback from the power transistor to produce dependable operation of the oscillator circuit when utilizing a full wave rectified unfiltered DC power source, and also aids in insuring oscillator operation at least ambient temperatures.

Alternating current voltages occurring at resistor 56 at the collector of driver transistor 14 are connected to the base of the power transistor 16 through a 0.22/50V. capacitor 60.

Biasing of the power transistor 16 forward in a conducting direction is produced through a divider network consisting of resistors 62 and 64 connected to the base of transistor 16 and the transistor emitter is connected to conductor 12 by conductor 66. Resistor 62 is of the temperature compensated type having a positive temperature coefficient and in the preferred embodiment, has a value of 100 OHM, 2W. Resistor 62, having a positive temperature coefficient, decreases in resistance with a decrease in temperature causing an increase in current to the base of transistor 16 forcing increased conduction at the lower temperature.

Resistor 64 of a conventional nature having a value of 100 OHM 1/2W. The resistors are connected in series across the direct current supply conductors l0 and 12 as indicated. The temperature compensated resistor 62 permits an increased flow of current through the resistor at lower temperatures, and thus sufficient bias will be maintained upon the base of transistor I6 under low temperature conditions to produce oscillation. At the higher ambient temperatures the current flowing through resistor 62 will be reduced as the temperature rises, and thus the power consumption of the circuit will be reduced insuring the most efficient utilization of power for any given temperature conditions. The low temperature starting features of the circuit overcome the inherent decrease in current gain common in silicone transistors as the temperature decreases.

In order to minimize the occurrence of interference signals generated by the circuit affecting nearby radio or television sets, a 0.0l MFD. 50V capacitor filter be installed in existing equipment by unskilled owners or installers, there is a possibility that the conductors and 12 could be erroneously connected to the existing polarized power supply conductors, and in the event the conductor 10 should be connected to ground,

and the conductor 12 connected to the positive supply source, the diode 72 will protect the circuit against damage.

When the conductors 10 and 12 are being supplied by a full wave rectified unfiltered direct current the voltage will fluctuate between zero and the maximum valve with each wave form and twice each cycle of the AC power source, i.e., every l/l th of a second when the rectifier is operated from a conventional 60 cycle AC source. Such a variation in the DC voltage will start and stop the oscillation of the power transistor 16 twice each cycle as a minimum voltage is required to maintain oscillation. In a conventional blocking oscillator circuit where the transistor output is divided between the load and feedback power to drive itself, the instability of the circuit when rapidly switched on and off when powered by a full wave rectified source produces inconsistent operation of the oscillator which may not always start each time the minimum voltage needed for operation is reached. However, in the described circuit the isolation of the feedback through the driver transistor I4 insures that the oscillation will start each time the rectified voltage reaches the minimum valve for operation. Thus, the feedback from power transistor 16 collector is isolated" from its base by the transistor 14 reducing any adverse effects within the power transistor itself. The feedback signal becomes a part of the output of driver transistor 14, rather than a part of the output of power transistor 16, resulting in improved reliability and stability in an oscillator circuit.

It will therefore be appreciated that the isolation of the feedback from transistor 16 through the driver transistor 14 provides a dependability of oscillator operation not achievable with conventional feedback systems, and this feature plus the incorporation of the temperature compensated resistor 62 in the biasing network for the power transistor permits dependable oscillator operation at lower temperatures. The heating of the lamp filaments 26 and 26 also aids in maintaining efficient lighting at lower temperatures, and the cir cuit results in dependable low voltage power supply operation of fluorescent lamps under conditions previously rendering the use of fluorescent light undependable and inefficient.

It is appreciated that modifications to the disclosed circuit may be apparent to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. An electrical circuit for fluorescent lamps supplied from a low voltage direct current power source characterized by its ability to efficiently function at low temperatures and when supplied by full wave rectified, pulsating, direct current comprising, in combination, a transformer having primary and secondary windings, a fluorescent lamp having first and second spaced electrodes, said secondary winding being connected to said first lamp electrode, a two conductor direct current power source, said second electrode being connected to said power source, a power transistor having a first base and a first interrupted current output, said output being connected to said primary winding, first and second resistors connected in series across said power source constituting a first divider network connected to said first base biasing said transistor toward a conducting state, a driver transistor having a second base and a second interrupted current output, third and fourth resistors connected in series across said power source constituting a second divider network connected to said second base biasing said second base toward a conducting state, and a feedback circuit interconnecting said first interrupted current output to said second base, said second interrupted current output being to said first base whereby said feedback circuit is isolated from said first base to insure reliable oscillation of said power transistor.

2. In an electrical circuit for fluorescent lamps as in claim 1 wherein said first resistor of said first divider network comprises a positive temperature compensated resistor.

3. In an electrical circuit for fluorescent lamps as in claim 2, a diode in series with the positive conductor of said direct current power source interposed in the circuit before said divider networks.

4. In an electrical circuit for fluorescent lamps as in claim 2 wherein said secondary winding includes terminal leads carrying the full potential output of said secondary winding, a capacitor voltage divider network connected across said terminal leads, a'pair of fluorescent lamps each having first and second electrodes said terminal leads each connected to a first electrode.

5. In an electrical circuit for fluorescent lamps as in claim 2, said fluorescent lamp second electrode comprising a second filament spaced from said first electrode and conductors connecting said second filament across said direct current power supply conductors maintaining said second filament in a heated condition during lamp operation.

6. In an electrical circuit for fluorescent lamps as in claim 2, wherein said secondary winding includes terminal leads carrying the full potential output of said secondary winding, a capacitor voltage divider connected across said terminal leads, a pair of fluorescent lamps each having first and second spaced electrodes and filaments, respectively, said terminal leads each' connected to a first electrode, and conductors connecting said second filaments in series across said direct current power source maintaining said second filaments in a heated condition during lamp operation.

7. In an electrical circuit for fluorescent lamps as in claim 6, a capacitor connected across one of said terminal leads and a direct current power supply conductor to produce simultaneous ignition of said lamps.

8. An electrical circuit for fluorescent lamps supplied from a direct current power source comprising, in combination, positive and negative direct current supply conductors, a transformer having, a primary winding having first and second leads and a secondary winding having third and fourth leads, a power transistor having a first base. a first collector and a first emitter, said first lead being connected to said positive conductor and said second lead being connected to said collector, said emitter being connected to said negative conductor, a

first positive temperature compensated resistor interconnecting said base and said positive conductor, a second resistor connecting said base and said negative conductor, a driver transistor having a second base, a second collector and a second emitter, a third resistor connecting said positive conductor to said second base, a fourth resistor connecting said negative conductor to said second base, a feedback conductor connecting said second lead to said second base through a first capacitor, said second collector being connected to said first base through a second capacitor, said second emitter being connected to said negative conductor, a fifth resistor interposed between said second collector and said positive conductor, a pair of fluorescent lamps each having first and second spaced electrodes and filaments, respectively, said third lead connected to one of said first electrodes and said fourth lead connected to the other of said first electrodes, said second electrodes being connected to said supply conductors, and third and fourth capacitors connected in series across said third and fourth leads, said negative conductor being connected intermediate said third and fourth capacitors.

9. In an electrical circuit for fluorescent lamps as in claim 8, a fifth capacitor interposed between said third lead and said positive conductor to insure simultaneous ignition of said lamps.

10. In an electrical circuit for fluorescent lamps as in claim 8, conductor means connecting said second filaments of said lamps in series across said positive and negative conductors to maintain said second filaments in a heated condition during lamp operation. 

1. An electrical circuit for fluorescent lamps supplied from a low voltage direct current power source characterized by its ability to efficiently function at low temperatures and when supplied by full wave rectified, pulsating, direct current comprising, in combination, a transformer having primary and secondary windings, a fluorescent lamp having first and second spaced electrodes, said secondary winding being connected to said first lamp electrode, a two conductor direct current power source, said second electrode being connected to said power source, a power transistor having a first base and a first interrupted current output, said output being connected to said primary winding, first and second resistors connected in series across said power source constituting a first divider network connected to said first base biasing said transistor toward a conducting state, a driver transistor having a second base and a second interrupted current output, third and fourth resistors connected in series across said power source constituting a second divider network connected to said second base biasing said second base toward a conducting state, and a feedback circuit interconnecting said first interrupted current output to said second base, said second interrupted current output being to said first base whereby said feedback circuit is isolated from said first base to insure reliable oscillation of said power transistor.
 2. In an electrical circuit for fluorescent lamps as in claim 1 wherein said first resistor of said first divider network comprises a positive temperature compensated resistor.
 3. In an electrical circuit for fluorescent lamps as in claim 2, a diode in series with the Positive conductor of said direct current power source interposed in the circuit before said divider networks.
 4. In an electrical circuit for fluorescent lamps as in claim 2 wherein said secondary winding includes terminal leads carrying the full potential output of said secondary winding, a capacitor voltage divider network connected across said terminal leads, a pair of fluorescent lamps each having first and second electrodes said terminal leads each connected to a first electrode.
 5. In an electrical circuit for fluorescent lamps as in claim 2, said fluorescent lamp second electrode comprising a second filament spaced from said first electrode and conductors connecting said second filament across said direct current power supply conductors maintaining said second filament in a heated condition during lamp operation.
 6. In an electrical circuit for fluorescent lamps as in claim 2, wherein said secondary winding includes terminal leads carrying the full potential output of said secondary winding, a capacitor voltage divider connected across said terminal leads, a pair of fluorescent lamps each having first and second spaced electrodes and filaments, respectively, said terminal leads each connected to a first electrode, and conductors connecting said second filaments in series across said direct current power source maintaining said second filaments in a heated condition during lamp operation.
 7. In an electrical circuit for fluorescent lamps as in claim 6, a capacitor connected across one of said terminal leads and a direct current power supply conductor to produce simultaneous ignition of said lamps.
 8. An electrical circuit for fluorescent lamps supplied from a direct current power source comprising, in combination, positive and negative direct current supply conductors, a transformer having a primary winding having first and second leads and a secondary winding having third and fourth leads, a power transistor having a first base, a first collector and a first emitter, said first lead being connected to said positive conductor and said second lead being connected to said collector, said emitter being connected to said negative conductor, a first positive temperature compensated resistor interconnecting said base and said positive conductor, a second resistor connecting said base and said negative conductor, a driver transistor having a second base, a second collector and a second emitter, a third resistor connecting said positive conductor to said second base, a fourth resistor connecting said negative conductor to said second base, a feedback conductor connecting said second lead to said second base through a first capacitor, said second collector being connected to said first base through a second capacitor, said second emitter being connected to said negative conductor, a fifth resistor interposed between said second collector and said positive conductor, a pair of fluorescent lamps each having first and second spaced electrodes and filaments, respectively, said third lead connected to one of said first electrodes and said fourth lead connected to the other of said first electrodes, said second electrodes being connected to said supply conductors, and third and fourth capacitors connected in series across said third and fourth leads, said negative conductor being connected intermediate said third and fourth capacitors.
 9. In an electrical circuit for fluorescent lamps as in claim 8, a fifth capacitor interposed between said third lead and said positive conductor to insure simultaneous ignition of said lamps.
 10. In an electrical circuit for fluorescent lamps as in claim 8, conductor means connecting said second filaments of said lamps in series across said positive and negative conductors to maintain said second filaments in a heated condition during lamp operation. 